The present invention relates to focus sensing and more particularly to a focus sensor for use with equipment where the exact distance between two devices is to be maintained, as for example, in a microscope or a optical recorder such as that disclosed in my co-pending application entitled MAGNETO-OPTIC REWRITTABLE OPTICAL HEAD, Ser. No. 07/771,042, filed Oct. 2, 1991, now U.S. Pat. No. 5,200,946 and assigned to the assignee of the present invention.
A method of determining whether light is collimated or is converging or diverging has been suggested and referred to as the "critical" angle method which is discussed on page 80 of a book entitled Principles of Optical Disc Systems by G. Bouwhuis et al. of Philips Research Laboratories, Eindhoven and published by Adam Hilger, Ltd., Bristol and Boston. More particularly, when a parallel beam of light passes through a prism of a first index of refraction and is incident at an angled surface just above the critical angle, the light is reflected. Two detectors may then be placed so that the radiation received by both detectors will be equal. If the incident beam is converging then a first portion of the beam will strike the surface below the critical angle and will not be reflected. This light will pass out of the prism but the light that is reflected from the remaining portion of the beam will strike one detector more than the other thus indicating convergence. Similarly, if the light is diverging, the other of the detectors will receive more energy than the first. By measuring the energy received by the detectors it may be determined whether the original beam was parallel, converging or diverging. This method has been suggested for use in focus sensing as shown by an article entitled "A Focus Sensing Method with Improved Pattern Noise Rejection" by D.R. Marshall of the Hewlett Packard Disk Mechanisms Division and published in the S.P.I.E. Volume 1499, Optical Data Storage, '91 on pages 332-339. In this technique light reflected from the surface is only parallel when the surface is at a first position (i.e. focused) and is either divergent or convergent on either side of the focus position. By sensing the output of two detectors in the "critical angle" method, the position of the pick up lens, which receives energy from the disk, can be maintained to assure proper focus.
A basic problem occurs with the focus sensing apparatus heretofore proposed in that the sensitivity of the system to even very slight angular changes of the medium through which the light passes can significantly alter the output. Likewise, even minor changes in the index of refraction which can occur as, for example, with changes in temperature or even changes in altitude make such systems extremely difficult to employ except in very controlled environments.
The present invention utilizes an effect associated with a thin homogeneous dielectric films such as described in the book "Principles of Optics" by Born and Wolf 5th ED Section 1.6 pp 61-67. According to this phenomena, light which passed through a homogeneous dielectric thin film situated between two homogeneous media will have a certain percentage reflected by the film and the rest transmitted through the film, assuming a no-losses non-absorbing film. The ratio of transmitted to reflected light at the film varies with incident angle, film thickness, optic index's of the film and surrounding media and the wavelength of the incident light.
One finds that for a thin film whose thickness is on the order of the wavelength of light used, with lower optic index than the surrounding media and with light incident nominally at the critical angle of the surrounding media/thin film, that the percentage of reflected light is highly sensitive to changes in the incident angle. Variations of reflection and transmission are described in an article entitles "A Variable Transmittance Beam Splitter" by R.N. Smartt, in the April 1970 issue of Applied Optics on pages 970-971.